U.S. Pat. Nos. 6,647,347, 6,878,931, and 7,129,480, issued to Roushall, assigned to Agilent Technologies, Inc., use a method of reducing noise that includes shifting the phase of an accumulation clock relative to a phase of a sampling clock. In this way, data acquisition is made to occur at an intermediate point of the clock period where ambiguities of signal values are avoided. In one embodiment, an additional accumulator path, which is also phase shifted, is included.
U.S. Pat. No. 7,084,395 to Fuhrer et al., and assigned to Ionwerks, Inc., discloses a time offset between ion generation and ion extraction. Fuhrer provides these time offsets between previous and subsequent runs. The spectrum is then reconstructed.
U.S. Pat. No. 6,831,280 to Scherer teaches synchronization of two signals for error correction.
U.S. Pat. No. 6,094,627 to Peck et al., U.S. Pat. No. 5,995,989 to Gedcke et al., and U.S. Pat. No. 6,028,543 also to Gedcke et al., each assigned to PerkinElmer Instruments, Inc. or related company EG&G Instruments, Inc., have digital signal averaging capabilities that can be implemented with an analogue to digital converter.
U.S. Pat. Nos. 6,822,227 and 6,744,044, and U.S. Patent Application No. 2004/0079877 all issued to Hidalgo, assigned to Agilent Technologies, Inc., disclose the general state of the art.
Improved accuracy in spectra has been obtained by increasing the number of shots taken of a sample in time of flight (TOF) instruments. In a digital approach, increased accuracy has been achieved by taking a large number of shots with a reduced number of ions being detected per shot. Then the data from the shots is summed to obtain a spectrum. This method may be implemented with a time to digital converter (TDC) device that substantially counts individual ions.
As used herein the term “sampling period” refers to the time during which a sampling or a sample of the data is taken. The sampling speed determines the period during which the sample data is collected. The term sample in this context is not to be confused with a sample of the type that comprises the compounds or materials to be analyzed in a mass spectrometer, for example. Rather, sample in accordance with the definition set forth in this paragraph and as predominantly used herein refers to the data or the signal representing that data as it is acquired in a single fundamental period of detection. This fundamental period of detection is the sampling period, and is the predetermined increment of time over which ions will be detected and their time values sent to the temporary storage or buffer without repetition.
As the number of ions per shot and per sample within each shot is further reduced, the physical limits of clocks are approached. This is because in order to further reduce the number of ions per sampling period, the sampling period is reduced. As a result, error is introduced due to the binary clock signal irregularities. Therefore, a device for correcting errors due to these irregularities is needed.